The present invention relates to television sound signal processing and, in particular, to a frequency translation phase-locked loop (PLL) for detecting the audio information.
Multichannel audio for stereo and bi-lingual broadcasting involves the use of one or more audio subcarriers, which subcarriers increase the television audio signal bandwidth from 15 kHz to approximately 90 kHz or more. As a result an audio buzz which is produced in the sound signal processing channel tends to be more severe.
Audio buzz, which may be defined as the result of picture-related modulation which is transferred to the audio signal, has always been present to some degree in television signal processing circuitry, but has been kept to within tolerable limits by various circuit techniques.
In the early days of television, "separate" or "split-sound" detection was used for detecting the audio information. The picture and sound signals were processed in separate channels which followed the tuner circuitry. Such separate processing prevented any significant interaction within the receiver of the picture and sound carriers and thus substantially eliminated receiver generated audio buzz. Unfortunately, any frequency variations of the tuner local oscillator due to drift or automatic fine tuning (AFT) were imparted to the intermediate frequency (IF) sound carrier as well as the IF picture carrier and were detected as spurious interference (buzz) by the audio frequency modulation (FM) detector. Additionally, since the passband of the sound channel is much narrower than the passband of the picture channel, the receiver had to be tuned for best sound, not best picture.
Modern day television receivers almost universally use the intercarrier method of sound signal processing. In the conventional intercarrier method, the picture and sound carriers are processed after the tuner in a common IF channel. This IF channel includes a particular IF bandpass response which (1) attentuates the sound carrier about 20 dB more than the picture carrier and (2) locates the picture carrier 6 dB down on the high frequency slope of the IF passband. Thereafter, the greater amplitude picture carrier is processed in the video channel for detecting the video information and, for recovering the audio information, the two IF carriers are mixed to form an intercarrier sound signal having a frequency corresponding to the difference of the carrier IF frequencies. E.g., for the NTSC system a 45.75 MHz picture carrier is mixed with a 41.25 MHz sound carrier for producing a 4.5 MHz intercarrier sound signal. The intercarrier sound signal is then separately demodulated (discriminated) for detecting the audio information.
The intercarrier detection method is particularly advantageous due to the fact that common mode FM of the sound and picture carriers caused, e.g., by tuner local oscillator variations in the receiver itself and in television accessories to which the receiver may be coupled, such as a cable TV converter, is cancelled by the IF signal mixing when the intercarrier sound signal is developed. However, the conventional intercarrier method is not buzz free. Although the above-noted intercarrier IF passband response is necessary for proper detection of the picture carrier; its use tends to increase audio buzz compared to the separate picture and sound channel method by: (1) significantly attenuating the sound carrier and thus reducing the signal-to-noise ratio of the audio signal, and (2) causing unequal IF picture carrier sideband attenuation which imparts an incidential carrier phase modulation (ICPM) to the picture carrier, commonly referred to as "Nyquist ICPM", which distorts the sound signal during intercarrier mixing. Additionally, in the conventional intercarrier detection system a high degree of modulation or overmodulation of the picture carrier (which is commonly caused by local affiliate stations insertion of picture overlays) can interrupt the picture carrier signal and cause audio buzz at the video line and field rates (i.e., 15,734 Hz and 60 Hz in the NTSC system).
The audio buzz produced by the intercarrier system, although present, can be tolerated in monophonic audio television receivers due to the relatively narrow passband of the sound channel in such receivers. Unfortunately, with the increased audio bandwidth of multichannel audio, the audio buzz is also increased and may no longer be within tolerable limits.
An audio detection system for processing television signals which may include a multichannel audio signal, in which the audio buzz can be reduced to within acceptable levels, is described in U.S. Pat. Application Ser. No. 412,902, U.S. Pat. No. 4,470,071, entitled, "TELEVISION SOUND DETECTION SYSTEM USING A FREQUENCY TRANSLATION PHASE-LOCKED LOOP" filed Aug. 30, 1982, in the name of Abraham E. Rindal and assigned, like the present application, to RCA Corporation. In that system audio information is detected by a frequency translation phase-locked loop (PLL) including a frequency translation signal mixer and a phase detector responsive to the IF picture and sound carriers, respectively, provided at the output of the tuner. More specifically, a picture bandpass filter and amplifier selects the picture carrier from the output of the tuner and applies it to an input of the signal mixer. A voltage controllable oscillator (VCO) applies a VCO signal to a second input of the mixer, and a frequency translated signal at the output of the mixer is applied to a first input of the phase detector. A sound bandpass filter and amplifier also coupled to the output of the tuner, selects the sound carrier and applies it to a second input of the phase detector. A lowpass filter coupled to the output of the phase detector which supplies a control signal to the VCO also provides the detected audio information. Since the frequency translation PLL processes the IF sound carrier without having it first pass through the above-noted intercarrier IF passband, picture-related audio buzz is significantly reduced from that which results from the conventional type of intercarrier detector. Furthermore, since the picture carrier is used to generate the frequency translation signal, which is used to detect the audio information, any common mode FM of the picture and sound carriers is cancelled by the phase detector. Consequently, in this respect, the frequency translation PLL can be considered a type of intercarrier detector.
Although common mode FM of the picture and sound carriers is cancelled in either the conventional type of intercarrier system or the PLL type of intercarrier system, if only the picture carrier had incidental FM, the very act of mixing the picture and sound carriers together would impress that incidental FM on the sound carrier and distort the FM encoded audio information. A particularly troublesome area in the television signal processing system wherein incidental FM may be impressed on the picture carrier, is at the television broadcast transmitter. Here, changes in the video and RF signal levels can result in changes in circuit parameters of the various parts of the transmitter and impart varying amounts of ICPM to the picture carrier. Additionally, the amount of ICPM of the picture carrier can vary depending upon the correctness of the transmitter adjustments and can therefore vary from station to station.
When there exists a high ICPM of the picture carrier only, it would be desirable to detect the sound signal without using the transmitted picture carrier, i.e., by using a "separate" type of audio detector. However, where there is substantial common mode FM, it would be desirable to use an intercarrier type of audio detector. Great Britain Patent Application No. 2089608 of Sony Corp., published June 23, 1982, describes a television receiver which includes two separate audio detectors, a conventional intercarrier type and a "separate" type, both coupled to the tuner output and operating continuously for detecting the audio signal. A switch coupled to the output of each audio detector controls which output will be used for deriving the audio information. In this system, two separate sound signal detection systems are required. This increases the complexity of the sound detection system, its power consumption and its cost.